1. Field of the Invention
The invention relates to an improvement in a shift control device of a vehicular automatic transmission that performs a second shift control by lowering the input shaft rotation speed through the engagement control of a friction engagement device in the case where during a first shift in a power-off state, a second shift judgment for a power-on downshift is made.
2. Description of the Related Art
There is a known vehicular automatic transmission that establishes a plurality of gear steps of different speed change ratios by selectively engaging a plurality of friction engagement devices and accordingly changes in speed the rotation transferred from the engine to the input shaft and outputs the speed-changed rotation. An example of such a device is described in Japanese Patent Application Laid-Open Publication No. HEI 8-244499, in which if during a first shift in a power-off state, a second shift judgment for a power-on downshift is made, a second shift control of engaging and disengaging friction engagement devices is performed to execute the second shift, and at the same time, the sharp rise of the rotation of the input shaft is prevented by reducing the torque of the engine through a throttle control, or an ignition timing retardation control, or the like. Incidentally, an ordinary single power-on downshift is performed by gradually releasing a release-side friction engagement device and gradually increasing the input shaft rotation speed. At the time of a multiplex shift caused by a power-off→on operation, however, the release-side friction engagement device is immediately released, and the shift is performed through the engagement control of the engagement-side friction engagement device.
Generally, in the power-on downshift, the input shaft rotation speed is once let to sharply rise to or above the synchronous rotation speed of the post-shift gear step, and the input shaft rotation speed in an upward tendency is lowered to the synchronous rotation speed to finish the shift. Therefore, while the aforementioned reduction of the engine torque is performed, the throttle valve opening degree or the like needs to be controlled to such an extent that the input shaft rotation speed sharply rises to or above the synchronous rotation speed. Therefore, there is a problem of a shift shock occurring as a large peak in the output shaft torque arises from inertia torque when the sharply raised input shaft rotation speed is lowered by the engagement of a friction engagement device.